A Group-III nitride semiconductor such as gallium nitride and the like is useful as a substance applicable to light-emitting devices such as light-emitting diode, laser diode, etc., and to high-frequency and high-power electronic devices such as HEMT, HBT, etc. Accordingly, needed is a Group-III nitride semiconductor substrate having good crystallinity and having a flat surface.
For example, regarding GaN substrates, those having, as the main face thereof, a (0001) plane (hereinafter referred to as +c plane) are the most popular at present. The main face as referred to herein means the face to form a device, or the broadest face of a substrate. The GaN substrate having a main face of +c plane is produced first by polishing the +c plane of a GaN crystal with abrasive particles, and then cleaning it with an acidic solution such as HF or the like, or a basic solution such as KOH or the like to remove the abrasive particles. The +c plane is extremely stable to various acids and alkalis, and therefore a GaN substrate having a flat and clean +c plane as the main face thereof is provided.
However, it is said that devices comprising a GaN substrate with a +c plane as the main face thereof have some problems. Specifically, InGaN-based blue or green LED and LD comprising a GaN substrate with a +c plane as the main face thereof have a problem in that a piezoelectric field forms in the direction of the growth axis, c-axis thereof. The piezoelectric field forms through piezoelectric polarization caused by deformation of the crystal structure of the InGaN layer, and owing to the polarization, the hole and the electron injected in the light-emitting layer separate from each other thereby reducing the recombination possibility that contributes toward light emission. As a result, a problem is pointed out in that the internal quantum efficiency lowers thereby resulting in the reduction in the external quantum efficiency (Non-Patent Reference 1).
Accordingly, for reducing the influence of the piezoelectric field, studies of InGaN-based blue or green LED and LD are being actively made, which have, as the growth surface thereof, a non-polar plane such as an a plane or an m plane vertical to the +c plane of the GaN crystal, or a semi-polar plane inclined relative to the +c plane. The a plane as referred to herein indicates a (11-20) plane and a plane equivalent thereto in a hexagonal crystal, and is concretely a generic term for (11-20) plane, (−1-120) plane, (1-210) plane (−12-10) plane, (−2110) plane and (2-1-10) plane. The m plane indicates a (1-100) plane and a plane equivalent thereto in a hexagonal crystal, and is concretely a generic term for (1-100) plane, (−1100) plane, (01-10) plane (0-110) plane, (10-10) plane and (−1010) plane.
However, in a case of a GaN substrate having, as the main face thereof, a semi-polar plane or a (000-1) plane (hereinafter referred to as −c plane) having a dangling bond density of more than 14.0 nm−2, the surface chemical reactivity is high, and therefore it has a problem in that, when cleaned with an acidic solution such as HF or the like or an alkaline solution such as KOH or the like, then its surface is roughened. For example, it is reported that the surface RMS of a GaN crystal, of which the (11-22) plane has been polished and cleaned, is 2.65 nm (Non-Patent Reference 2).
Non-Patent Reference 1: Nikkei Electronics, 14.8.2006, pp. 65-70,
Non-Patent Reference 2: Seminar in the Kansai Branch of the Japan Society of Applied Physics, “Crystal Growth and Device Application”, Mar. 12, 2007.